Regulatory assets and liabilities Regulatory Liabilities (Details 2) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 60,072	$ 42,935	$ 61,390
Regulatory Liability, Noncurrent	443,205	447,370	457,084
Total regulatory liabilities	503,277	490,305	518,474
Net regulatory position	(99,882)	(72,908)	(124,861)
Natural gas costs refundable through rate adjustments
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	32,181	23,825	29,739
Taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,002	3,472	8,139
Regulatory Liability, Noncurrent	240,148	246,034	259,638
Plant removal and decommissioning costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	174,120	173,722	172,569
Pension and postretirement benefits
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	18,040	18,065	15,239
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	23,889	15,638	23,512
Regulatory Liability, Noncurrent	$ 10,897	$ 9,549	$ 9,638